Nitrogen recovery system and method using heated air as stripping gas

ABSTRACT

A process for the recovery of nitrogen from anaerobically digested liquid waste and for the collection of the nitrogen as nitrate compounds that can be used to produce fertilizer and compost, includes stripping ammonia from anaerobically digested liquid waste, and converting the ammonia into nitrates via nitrification. The stripping gas is heated above ambient atmospheric temperature to improve nitrogen recovery. The heat can be reclaimed by burning gases generated during the anaerobic digestion process.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/826,131,filed Apr. 16, 2004, now U.S. Pat. No. 7,153,427, which is acontinuation-in-part of application Ser. No. 10/625,198, filed Jul. 22,2003, now U.S. Pat. No. 6,866,779, which claims the benefit ofProvisional Application No. 60/398,296, filed Jul. 22, 2002. All theabove are incorporated herein expressly by reference in their entirety.

FIELD OF THE INVENTION

The field of the invention is related to the removal and recovery ofnitrogen from anaerobically digested liquid waste and the collection ofthe nitrogen as nitrates and organic nitrogen compounds that can be usedto produce fertilizer and compost.

BACKGROUND OF THE INVENTION

In U.S. application Ser. No. 10/625,198, fully incorporated herein byreference in its entirety, a method is described that is related to therecovery of nitrogen from ammonia that is produced from anaerobicdigestion of waste. In the prior process, atmospheric air at ambienttemperature is used as the stripping gas. However, if ambient air isused, especially during winter, the temperature of the digested liquidwaste can be significantly reduced to a temperature, such as 40° F. to50° F., that will lower the amount of ammonia and hydrogen sulfide thatis stripped from the digested liquid waste. The lower temperature willalso negatively affect the efficiency of the biofilter that fixes thenitrogen into a solid form.

Accordingly, there is a need to improve on the prior method to moreefficiently recover ammonia, and subsequently nitrogen. The presentinvention overcomes the drawbacks of the prior method and has furtherrelated advantages.

SUMMARY OF THE INVENTION

The present invention is related to a process for the recovery ofnitrogen from wastewater liquids, and collection and sequestration ofthe nitrogen in organic matter taking the form of inorganic nitrogen andorganic nitrogen compounds. Recovered nitrate, ammonia, and organicnitrogen compounds can be used to produce nitrogen-rich organic compostfertilizer. One embodiment of the method according to the invention canrecover nitrogen from liquid waste to produce nitrogen-rich fertilizer.The process produces less greenhouse gases as compared with the typicalnitrification and denitrification processes. Such reduction in ammonialevels and greenhouse gases is thought to produce beneficial healthbenefits for people and lessen the environmental impact.

In one embodiment of the present invention, a method for recoveringnitrogen from liquid waste is provided. The method includesanaerobically digesting liquid waste in an anaerobic digester. Throughthis process, digested liquid waste is produced containing amounts ofdissolved carbon dioxide, hydrogen sulfide, and ammonia. Dissolvedammonia can exist in liquid as ammonium ions. The digested liquid wastecan be stripped of carbon dioxide and ammonia with a heated gas, such asheated air that is heated above the ambient atmospheric temperature, orcombustion gas. Upon stripping the carbon dioxide, the ammonium ionsform ammonia gas and the ammonia gas is likewise stripped from thedigested liquid waste. The gases leaving the stripping unit can be fedto a biofilter containing bacteria that convert the stripped ammoniainto nitrate compounds and bacterial biomass, for example.

In one embodiment of the invention, the heated stripping gas is producedby collecting the combustible gases, such as methane, which aregenerated during the digestion process, and burned. The burning of themethane can be used in power generation equipment. Alternatively, themethane and other combustible gases can be burned in a flare stack. Heatto heat stripping air can be reclaimed during the burning process viaseveral methods. For example, a heat exchanger can be provided toreclaim heat from various hot media, such as hot water, condensate,steam, and combustion gas. Alternatively, the combustion gas producedduring burning can be used as the stripping gas. However, combustion gaswill contain an excess amount of carbon dioxide that will lower the pHof the digester effluent and therefore favor the removal of hydrogensulfide rather than ammonia nitrogen. Stripped hydrogen sulfide can beconverted into sulfur-containing compounds in a biofilter. Heatedstripping gas can be collected in the form of heated ambient atmosphericair from a building housing the power generation equipment.

Using heated air as the stripping gas will increase the removal ofcarbon dioxide and ammonia. Using heated air will also increase thetemperature of the biofilter, thus increasing moisture content (90+%saturation from stripping), biological activity, and nitrogen and/orhydrogen sulfide fixation into solid compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of one embodiment of a methodaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates one embodiment of a method according tothe present invention. Anaerobically digesting liquid waste 10, such asflush water containing cow manure from dairy farms, is followed by gasstripping or desorption 28 of dissolved carbon dioxide and ammonia fromthe digested liquid. The stripped gases are treated in abacteria-activated “biofilter” 38, that converts ammonia into nitratecompounds and other nitrogenous compounds that can then be used asfertilizer. The method according to the invention combines the processesof anaerobic digestion, desorption, and nitrification to advantageouslyrecover nitrogen, which would otherwise be discharged into theatmosphere.

The system to carry out the method according to the present invention,includes an anaerobic digester 10, a gas desorption or stripping unit28, a nitrification absorption unit or biofilter 38, and optionally asolids separator 18. It is to be appreciated that the figure representsa portion of what may be a larger integrated system. For example, theinfluent liquid waste 12 may have undergone pretreatment before arrivingat the anaerobic digester 10, such as by being processed through severalscreens and sedimentation, or holding ponds. However, pretreatment ofthe waste in this manner may also potentially reduce the amount ofsolids that can be converted into valuable products. These predigesterunit operations are described in Dairy Waste Anaerobic DigestionHandbook, by Dennis A. Burke (2001), incorporated herein by reference inits entirety. The Handbook is available on the Internet atwww.makingenergy.com. The system according to the invention may beintegrated into existing dairy farms to recover the nitrogen that isotherwise lost into the atmosphere through the conventional process oftreating water generated by dairy operations.

According to the invention, the anaerobic digester 10 digests theinfluent liquid waste stream 12. The influent liquid waste 12 caninclude, but is not limited to, water, collected during the typicaloperation of a dairy farm. Anaerobic digestion 10 produces digestedliquid waste 14 through the breakdown of organic material via amicrobial population that lives in the oxygen-free environment in thedigester 10. When organic matter decomposes in an anaerobic environment,the bacteria produce, at a minimum, methane and carbon dioxide gas.Nitrogen-containing compounds are converted to ammonia, andsulfur-containing compounds are converted into hydrogen sulfide. In thesystem according to the invention, these gases can be vented from thedigester 10 via vent line 22. Some gases are soluble and can remaindissolved within the liquid. Vented methane gas can be used as an energysource. Methane, and other combustible gases, produced during digestion,and drawn from vent line 22 can be routed to power generation, block 44.For example, boilers can burn the methane gas to produce steam that canbe used to drive turbines. Heat can be recovered from the boiler vianumerous methods. Forms of power generation include steam turbines,methane engines, and fuel cells. Each power generation method producessome amount of heat that can be reclaimed as heated stripping gas.Alternatively to power generation, the methane and other combustiblegases can be disposed of by flaring to a stack. Heat can be reclaimed bythis later method as well.

The digested liquid waste 14, leaving the digester 10, includes water,soluble organic and inorganic compounds, such as soluble gases, andinsoluble organic and inorganic compounds. Reference is again made tothe Dairy Waste Anaerobic Digestion Handbook for a more detaileddescription of anaerobic digestion. In particular, representativeexamples of suitable anaerobic digesters for use in one embodiment ofthe method according to the present invention are described therein,including, but not limited to, covered anaerobic lagoons, plug flowdigesters, mesophilic completely mixed digesters, thermophiliccompletely mixed digesters, anaerobic contact digesters, and hybridcontact/fixed film reactors, and the like. Such digesters are at leastsuitable for processing dairy waste. Other anaerobic digesters includepacked fixed film reactors, upflow anaerobic sludge blanket reactors(UASB), and horizontal baffled reactors, described also in the Handbook.

After anaerobic digestion 10, the liquid waste 12 contains carbondioxide, ammonia, hydrogen sulfide, and other dissolved inorganiccomponents, such as alkaline compounds. Typically, the majority ofammonia remains dissolved in digested waste liquid 14, as the ammoniumion. The pH of digested liquid waste 14 can be between about 7 and about8. The anaerobic digestion process also produces carbon dioxide thatdissolves in the digested liquid waste 14. The amount of dissolvedcarbon dioxide is a function of at least the partial pressure of carbondioxide, which is typically between about 25% to about 40% of the totalpressure within the anaerobic digester 10. In accordance with thepresent invention, the anaerobic digestion process can be used toproduce a digested liquid product, which has a substantial amount ofcarbon dioxide and ammonia dissolved therein. The anaerobic digestionprocess also uses heat through the thermophilic or mesophilic digestionof the liquid waste influent 12. Since the subsequent process of ammoniastripping is both temperature and pH dependent, higher digestiontemperatures and pH values in the anaerobic digester 10 are advantageousin the stripping process to recover ammonia. Lower pH values areadvantageous in the stripping process to recover hydrogen sulfide.Higher pH values are achieved by stripping carbon dioxide, lower pHvalues are achieved by carbon dioxide through the stripping process byusing a high carbon dioxide content gas. Both low and high pH values toremove hydrogen sulfide and ammonia can be achieved by first strippingwith the combustion gas followed by stripping with air.

In one embodiment of the present invention, a liquid/solid separator 18can be provided after anaerobic digestion 10. The separator 18 is anoptional piece of equipment. Separation is optional, according to oneembodiment of the invention. Digested liquid waste 14 includes a solidphase and a liquid phase. The separator 18 is provided to separate thesolid phase from the liquid phase. The separated solid phase 20 can berecycled to the anaerobic digestion process 10. Additionally, oralternatively, the separated solid phase 20 can be diverted to otheroperations, which process the solid phase into compost or otherbeneficial products 24, or alternatively used in biofilter 38. The solidphase will contain some nitrogen compounds and the majority of thephosphorus. Digested liquid waste having reduced solids is representedby reference numeral 16. Representative separators 18 include filters,screens, screw presses, flotation, and gravity separators. A suitableflotation separator is described in U.S. application Ser. No.10/194,451, filed Jul. 11, 2002, incorporated herein by reference in itsentirety. Solid-phase separation methods and units are explained in manyengineering textbooks, such as the Chemical Engineers' Handbook, 5thed., by Perry and Chilton, incorporated herein in its entirety byreference. A stripping unit 28 to recover ammonia follows the separator18 or, if no separator 18 is provided, the anaerobic digester 10.

According to the present invention, digested liquid waste, containingsolids 14 or reduced solids 16, is stripped of at least some ammonia,carbon dioxide, or hydrogen sulfide in a stripping unit 28. As usedherein, digested liquid waste can refer to digested liquid wastecontaining solids 14, or digested liquid waste with reduced solids 16 ifthe optional separator 18 is used prior to stripping 28. The strippingunit 28 receives digested liquid waste 14 or 16, containing at a minimumdissolved carbon dioxide, dissolved hydrogen sulfide and dissolvedammonia. Carbon dioxide, hydrogen sulfide and ammonia (acting as thesorbate) can be stripped from the liquid by contact with a stripping gas30 (acting as the sorbent) through a process known as desorption. Thestripping gas 30 is preferably heated above the prevailing ambientatmospheric temperature. In one embodiment of the present invention, theheated stripping gas 30 is produced in the power generation step, block44. Heat produced during power generation can be reclaimed by employingvarious methods. For example, the combustion gas that is produced duringburning of the methane, or other combustible gas produced via thedigestion process, can be used as the stripping gas 30. Combustion gasused in stripping can have a carbon dioxide content of at least 30% byvolume and at least 5% oxygen by volume. The temperature of the heatedstripping gas 30 in this instance can be about 350° F. or greater.Combustion gas contains higher amounts of carbon dioxide as comparedwith atmospheric air that will tend to lower the pH. A lowering of thepH favors the removal of hydrogen sulfide gas as compared with ammoniagas. This is due, it is believed, to the hydrogen sulfide ions insolution (HS⁻) gaining a hydrogen ion (H⁺) under reduced pH conditions,and thus increasing the amount of hydrogen sulfide (H₂S) in solution.Heat can also be reclaimed using heat exchangers wherein ambientatmospheric air can be directed to pass on one side of the heatexchanger and hot water, condensate, steam, or combustion gas is passedon the opposite side of the heat exchanger to thus transfer heat fromthe hot medium to the relatively colder ambient air. The temperature ofair is thus heated above the ambient temperature. The temperaturesachieved by heat exchangers can be as much as about 200° F. or greater.Alternatively, a power generator can be housed in a building whereby theambient air surrounding the power generation equipment is heated viaradiant and convection means. The air from the building can be collectedand used as the heated stripping gas 30. Air heated via this latermethod can be about 80° F. to about 110° F. Heated stripping gas 30produced from radiant, convection, and conduction sources, such asheated atmospheric air, will be lower in carbon dioxide as compared withcombustion gas, and thus will be more efficient at removing carbondioxide. The reduction in carbon dioxide will thus increase the pH ofthe digested liquid waste and improve ammonia recovery. In oneembodiment of the present invention, therefore, the pH of the digestedliquid waste may first be lowered by stripping with heated air,effectively removing carbon dioxide and ammonia, followed by a secondstripping process using combustion gas, effectively lowering the pH, andstripping hydrogen sulfide from the digested liquid waste. However, inother embodiments, stripping hydrogen sulfide with combustion gas can befollowed by stripping ammonia with heated air. In yet other embodiments,heated air or combustion gas can be employed as the sole stripping gas30. Stripping with heated air raises pH, removes acidic gases, such ascarbon dioxide, from the digested liquid waste (with or without solids)for ammonia removal, while stripping with combustion gas lowers pH, addsacidic gases, such as carbon dioxide, to the digested liquid waste (withor without solids).

The stripping gas 30 can contain comparatively lower amounts of carbondioxide and ammonia as compared with the digested liquid waste 14 or 16,so as to provide a concentration gradient that will cause the diffusionof the carbon dioxide and ammonia from the digested liquid waste 14 or16 into the stripping gas 30. It is to be appreciated that moleculardiffusion is but one process that can be occurring to cause the carbondioxide and ammonia to transfer into the stripping gas 30. As desorptionof carbon dioxide takes place, the pH of the digested liquid waste 14 or16 may increase due to the removal of the carbon dioxide. As the pH ofthe digested liquid waste increases, some of the ionized ammonia(ammonium ions) will be converted into gaseous ammonia. A portion ofthis ammonia may be stripped in conjunction with the carbon dioxide.Eventually, the carbon dioxide concentration of the digested liquidwaste 14 or 16 will be reduced, such that the concentration of carbondioxide approaches equilibrium with the concentration of the carbondioxide of the stripping gas 30. At this point, the pH of the digestedliquid waste 14 or 16 can be between about 8 to about 9. At this pH, theammonium ions tend to form into ammonia and are removed by the strippinggas. As the gaseous ammonia is removed, ammonium ions continue to beconverted to gaseous ammonia. The fundamentals of desorption orstripping is explained in many engineering books, such as the ChemicalEngineers' Handbook, 5th ed., by Perry and Chilton, pp. 14-2 to 14-16,incorporated herein by reference, in its entirety. Typically, packedtowers or plate towers are used to carry out gas desorption fromliquids, each type of tower having its advantages and disadvantages,depending on the ultimate application. The engineering literature hasfuller descriptions of suitable stripping towers. For example, referenceis made to the United States Environmental Protection Agency Paper, EPA832-F-00-019 (September 2000), incorporated herein by reference in itsentirety.

As an alternative embodiment, caustic, or alkaline chemicals 26, may beadded to the digested liquid waste 14 or 16, to raise or lower the pH toassist in the stripping of ammonia or hydrogen sulfide from the digestedliquid waste. As shown in the figure, alkalinity can be added to thedigested liquid waste 14 or 16 either before the stripping unit 28 or tothe stripping unit 28. The amount of alkalinity can be varied. Asuitable amount of alkalinity to add can be obtained by balancing theconsumption of energy required in the stripping process and the cost ofalkaline chemical addition. A suitable alkaline compound can be sodiumor calcium hydroxide, and the like. Magnesium hydroxide or magnesiumoxide can also be added, in this case, before or after anaerobicdigestion.

As another alternative embodiment, the temperature of the digestedliquid waste 14 or 16 can be increased. Increasing the temperature ofthe digested liquid waste 14 or 16 will result in an increased rate ofstripping the ammonia from the liquid. A heat exchanger can be providedin the line to the stripping unit 28 or, additionally or alternatively,the stripping unit can be provided with a jacket surrounding thestripping vessel and so provide for heat exchange between acomparatively hot fluid and the digested liquid waste 14 or 16.Heat-providing media 32, such as steam, or other condensable fluids, orhot liquids, can be introduced directly, or alternatively, on the shellor tube side of a heat exchanger or the jacket of the stripping vessel28. The temperature of the digested waste liquid 14 or 16 may be variedaccording to the desired amount of stripping performance. The system canreadily be provided with heat transfer equipment to provide heat, inaddition to the heat that is produced during anaerobic digestion.

Increasing the pH of the digested liquid waste 14 or 16 by introducingalkalinity to the digested liquid waste 14 or 16, either before thestripping unit 28 or in the stripping unit 28, and raising thetemperature of the digested liquid waste 14 or 16, is believed toincrease the rate at which ammonia can be stripped from the digestedliquid waste 14 or 16. The stripping unit 28 can produce a strippedliquid-phase component 34 having reduced quantities of soluble compoundsand a gas-phase component 36 containing the soluble compounds.

Decreasing the pH of the digested liquid waste 14 or 16 can be achievedby adding acidity. Such pH reduction can come about through theintroduction of acidic compounds, such as but not limited tohydrochloric, sulfuric, and phosphoric acids. The stripped liquid waste34 is discharged from the stripping unit 28 and may be processedfurther. Stripped gas 36 from the stripping unit 28, contains, at aminimum, ammonia hydrogen, sulfide, or both. A nitrification biofilter38, located downstream of the stripping unit 28, can be used to convertthe ammonia that is stripped from the digested liquid waste 14 or 16into nitrogenous compounds, other than ammonia, by bacterial activity.Nitrate compounds are the result of a process referred to asnitrification. The literature is replete with descriptions of thenitrification process that oxidizes ammonia into nitrite by Nitrosomonasbacteria, and from nitrite to nitrate by Nitrobacter bacteria. Thenitrate compounds 40 can be collected, and further processed, and/orrefined into desirable products, such as nitrogen-rich compost, orfertilizer. The gas-phase component 42 can be discharged from thebiofilter 38.

A nitrification biofilter 38 according to the invention may include afibrous material, such as compost or a synthetic porous media capable ofsupporting a bacterial consortium for the conversion of ammonia tonitrate compounds and organic biomass containing nitrogen. The biofilter38 can have sufficient nutrient value to support the bacterialconsortia. Additionally or alternatively, nutrients and micronutrientsmay be added to ensure adequate bacterial performance. Moisture may alsobe added, if sufficient moisture is not already present in the gas 36from the stripping unit 28. The quantity of moisture present in the gas36 may be a function of the temperature at which the digestion andstripping phases are carried out. According to the present invention,the biofilter can support a bacterial consortia that will absorb andprecipitate the ammonia gas into nitrate compounds and/or organicbiomass containing nitrogen. Depending on the type of biofilter used,the biofilter media 40 can be replaced from time to time with new media.During the replacement of the biofilter media, the newer media may beseeded with bacterial consortia of the previous filter media. The filtermedia that are removed will be substantially higher in nitrogencompounds than the original filter media.

Biofilters are presently being used to eliminate odors from buildings,such as barns, by venting the barn air through a bed of organicmaterial. As the air passes through the organic medium, microorganismsconvert the organic gases into carbon dioxide and water. Literature onbiofiltration regarding the recovery of nitrogen from ammonia is alsoavailable. For example, reference is made to the articles, “Biofiltersfor Odor Control,” D. Schmidt, et al., University of Minnesota, and“Gaseous Ammonia Removal in Biofilters: Effect of Biofilter Media onProducts of Nitrification,” J. A. Joshi et al., Rutgers, The StateUniversity of New Jersey, both incorporated herein by reference in theirentirety.

Alternatively, aerobic biofiltration can be utilized to convert hydrogensulfide into sulfuric acid or elemental sulfur. Hydrogen sulfide gas isreadily soluble in water. Thus, any hydrogen sulfide in the stripped gas36 can be removed with the use of an aerobic biofilter.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method for recovering nitrogen from liquid waste, comprising: (a)anaerobically digesting liquid waste into digested liquid wastecontaining ammonia; (b) stripping said ammonia from said digested liquidwaste to produce gas containing ammonia; and (c) converting strippedammonia into nitrogen-containing compounds in a biofilter.
 2. The methodof claim 1, further comprising separating solids from the digestedliquid waste prior to stripping.
 3. The method of claim 1, furthercomprising raising the pH of the digested liquid waste.
 4. The method ofclaim 1, further comprising raising the temperature of the digestedliquid waste.
 5. The method of claim 1, wherein ammonia is stripped bycontacting the digested liquid waste with air.
 6. The method of claim 1,further comprising collecting organic biomass containing nitrogen. 7.The method of claim 1, wherein the pH of the digested liquid waste isabout 7 to about
 8. 8. The method of claim 1, wherein the pH of thedigested liquid waste is about 8 to about
 9. 9. The method of claim 2,wherein the separated solids are recycled to anaerobic digestion.
 10. Amethod for recovering nitrogen from anaerobically digested liquid waste,comprising: (a) stripping ammonia from anaerobically digested liquidwaste to produce gas containing ammonia; and (b) converting strippedammonia into nitrogen-containing compounds in a biofilter.
 11. Themethod of claim 10, further comprising raising the pH of the digestedliquid waste.
 12. The method of claim 10, further comprising raising thetemperature of the digested liquid waste.
 13. The method of claim 10,wherein ammonia is stripped by contacting the digested liquid waste withair.
 14. The method of claim 10, further comprising collecting organicbiomass containing nitrogen.
 15. The method of claim 10, wherein the pHof the digested liquid waste is about 7 to about
 8. 16. The method ofclaim 10, wherein the pH of the digested liquid waste is about 8 toabout
 9. 17. A method for making fertilizer from liquid waste containingmanure, comprising: (a) anaerobically digesting liquid waste containingmanure into digested liquid waste containing ammonia; (b) stripping saidammonia from said digested liquid waste to produce gas containingammonia; and (c) converting stripped ammonia into nitrogen-containingcompounds in a biofilter.
 18. The method of claim 17, further comprisingseparating solids from the digested liquid waste prior to stripping. 19.The method of claim 17, further comprising raising the pH of thedigested liquid waste.
 20. The method of claim 17, further comprisingraising the temperature of the digested liquid waste.
 21. The method ofclaim 17, wherein ammonia is stripped by contacting the digested liquidwaste with air.
 22. The method of claim 17, further comprisingcollecting organic biomass containing nitrogen.
 23. The method of claim17, wherein the pH of the digested liquid waste is about 7 to about 8.24. The method of claim 17, wherein the pH of the digested liquid wasteis about 8 to about
 9. 25. The method of claim 17, wherein the separatedsolids are recycled to anaerobic digestion.